Yes, the only place you can check to see how many days or plusses are left is at a Ticket Sales or Guest Relations window in the parks, or at the front desk of the Disney resorts (the resorts didn't do this for you up until last year).
A Plus option on an old ticket is an admission that doesn't expire. You can use those old Plusses to get into a WDW water park or Pleasure Island without paying any extra, no matter how many tiems the prices have gone up since you bought the ticket. FYI, however, the old Plus options from before the MYW system were not valid at DisneyQuest.
Your old tickets will work in teh turnstiles just fine, but you will be required to use the biometric scanners, just like the new tickets. The biometric finger data will be stored in teh WDW computer system and prevent anyone else from using the ticket after that, so take a sharpie with you and write down on the tickets how many Days and Plusses they each have left, and who they belong to so they don't get mixed up.